EP0580579A4

EP0580579A4 - Improvements in and relating to transmission line loudspeakers

Info

Publication number: EP0580579A4
Application number: EP19910920600
Authority: EP
Inventors: Khosrow Eghtesadi; William John Joseph Hoge
Original assignee: Noise Cancellation Technologies Inc
Current assignee: Noise Cancellation Technologies Inc
Priority date: 1991-04-19
Filing date: 1991-04-19
Publication date: 1994-06-15
Anticipated expiration: 2011-04-19
Also published as: JPH06508445A; DE69129664T2; EP0580579A1; WO1992019080A1; CA2108696A1; ES2118093T3; EP0580579B1; DK0580579T3; DE69129664D1; HK1011163A1

Abstract

The sound wave radiated from the front surface (UF) of a loudspeaker driver diaphragm is of opposite polarity with respect to that radiated from the back surface (UB). If the two signals are directly combined, they will tend to cancel one another. An acoustic phase inversion network (5) is used to ensure that the backwave is in phase with the front wave, and the combined signals are used to drive the inlet of a loudspeaker transmission line (6).

Description

improvements In and Relating to Transmission Line Loudspeakers

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a method of and apparatus for improving the performance of transmission line loudspeakers and has particular applications in active noise control, high fidelity audio, and sound reinforcement systems.

2. Discussion of the Relevant Art.

The simplest form of a loudspeaker system is the direct radiator. Such a loudspeaker radiates sound directly form the enclosure aperture(s)—the driver diaphragm and, in the case of vented-box systems, a vent or port. There are no additional devices through which the sound passes.

A transmission line loudspeaker adds an additional device, such as a horn for impedance matching, through which some or all of the sound passes. Prior forms of transmission line systems may be divided into three classes. A type A transmission line system consists of a closed-box direct radiator loudspeaker with a transmission line added to the driver aperture. All radiated sound passes through the transmission line. A Type B or C system consists of a direct radiator system with the transmission line coupled to the back chamber of the enclosure. Both the Type B or Type C form exhibit the fault that the transmission line presents an acoustical short circuit to the back of the driver at least at some frequencies. This can cause serious dips in the system response.

SUMMARY OF THE INVENTION This invention relates to an improved form of the Type A System in which the signals from both the drive aperture and the port of a vented-box direct radiator system are combined to drive the transmission line. The original form of the Type A system prevents the back wave from the drive diaphragm from interfering with the front wave by trapping the back wave in the closed cavity behind the driver. The improved system passes the back wave trough an acoustic phase inverter so that it may be combined with the output from the front side of the driver. This doubles the energy available to drive the transmission line. This improvement should not be confused with hybrid systems which used a vented-box system with the transmission line coupled to either the driver aperture or the vent but not both.

The improved performance is roughly analogous to that seen in a vented-box direct radiator system as compared to a closed- box system. Either the efficiency or the bandwidth may be increased; or the system size may be decreased; or a tradeoff may be made among these possible benefits.

In one arrangement of the apparatus according to the present invention one or more electrodynamic loudspeaker drivers is installed in a vented-box direct radiator enclosure, and a cover is added to the front of the enclosure. This cover forms a front chamber into which both the driver and the vent radiate sound. The sound passes through the front chamber and into the transmission line. For applications requiring high efficiency over a wide bandwidth the transmission line should be a horn. However, a compact system might use a short tube. Such a tube is too short to exhibit transmission line characteristics. It will act as lumped parameter acoustic mass instead.

The transmission characteristics of the improved system using a horn will be determined in great measure by the horn and the acoustic load it presents to the front chamber, but will, in general, be high-pass in nature. The transmission characteristics of the short tube system will be band-pass in nature, the driver and the vented-box portion of the enclosure will provide a 4-pole high-pass response, and the front chamber and the outlet tube will provide a 2-pole low-pass response.

Another arrangement of the apparatus according to the present invention is particularly useful for active noise cancellation applications such as exhaust mufflers or duct silencers. In this arrangement the pipe or duct through which the noisy signal is flowing passes through the enclosure and exists through the outlet tube. The end of the noisy pipe or duct is aligned with the end of the loudspeaker and the two are coaxial. Thus, the antinoise signal radiated by the loudspeaker during the active cancellation is coaxial with the noise. Very good cancellation may be obtained at frequencies with wavelenghts which are long compared to the size of the outlet.

Another arrangement of the apparatus according to the current invention whicfe also has particular application in active noise cancellation systems is similar to that described immediately above. However, in this arrangmeent the pipe or duct containing the noisy flow does not pass through the loudspeaker. Instead, the loudspeaker outlet tube connects the loudspeaker front chamber to the pipe as a tee fitting into the pipe. In this case, the pipe need not end at the point where the noise and antinoise are mixed. This arrangement is useful for "in duct" cancellation.

The invention will now be further described by way of examples, with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 3 are signal flow graphs of the improved loudspeaker with a long transmission line and a short outlet tube,

FIGS. 2 and 4 are simplfied drawings of the invention, and FIGS. 5 to 9 show two ways Tn which the invention may be put to practical use in noise cancellation applications.

FIG. 10 shows a general form of a vented box bandpass loudspeaker.

FIG. 11 shows a simplified acoustical analagous circuit. Description of the Preferred Embodiments

Consider first FIGS. 1 and 2. FIG. 2 shows an electrodynamic loudspeaker driver 1 mounted in an enclosure 2 so that one side of the driver diaphragm radiates sound into the fornt chamber of the enclosure 3. The sound form the other side of the driver passes through the acoustic phase inverter comprising the back chamber 4 and the inner vent 5 which connects the front and back chambers. The total system output consists of the sum of the front wave and the phase corrected back wave flowing through the front chamber and out via the transmission line 6.

FIG. 1 shows the basic signal flow graph of the system using an electroydnamic driver 1. Electric potential E„ is applied accross the driver voice coil which has a resistance R_E and a resulting current I_vc flows. The electrodynamic coupling Bl of the motor causes a driving force. The sum of this force and the various reaction forces in the system gives in the total force driving the diaphragm F_D. This force accelerates the diaphrgam at a rate inversely proportional to the moving mass _MS of the driver. The resulting acceleration of the diaphragm a_D is integrated once with respect to time (the 1/s operation in the LaPlace domain) to find the velocity of the diaphragm u_D and a second time to find the displacement of the diaphragm x_D. Now, moving the diaphragm results in some reaction forces. As the diaphragm is displaced against the mechancial springs in its suspension, an opposing force inversely proportional to the mechanical compliance C_MS of the driver is added to the total force F_D. Another opposing force results from the motion through the mechanical losses R_MS of the system and is equal to the product of R_MS and u_D. Also, as the voice coil moves through the magnetic field of the motor a back emf is generated which tends to oppose the driving potential. This back emf, which is equal to the electromagnetic coupling Bl times the diaphragm velocity u_D, sums with the input potential Eg to give the voice coil potential Eγ_C.

As the diaphragm moves, the front side pushes against the surrounding air and a flow into the front chamber 3 results.

This volume D_D is equal to the product of the diaphragam velocity u_D and its effective area S_D. This volume velocity is one of the components of the total flow into the from chamber Up. The conservation of matter requires that the flow into the back chamber U_β across the boundary between it and the front chamber be equal to Up but opposite in polarity. The volume velocity u_β presurizes the back chamber 4. The acoustic pressure of the back chamber p_B is equal to the integral of U_B with respect to time divided by the acoustic compliance to the back chamber C^g. This pressure exerts another reaction force against the back of the diaphragm which is equal to the pressure p_B times the diaphragm area S_D. This another component of F_D.

For the purpose of an orderly description of the system, assume that the inner vent 5 is blocked. This is equivalent to the unimproved form of the transmission line loudspeaker. The flow into the from chamber 3 pressurizes it. This component of the front chamber acoustic pressure _F is equal to the integral of Up with respect to time divided by the acoustic compliance of the front chamber C^p. The front chamber pressure drives the flow through the transmision line 6 at a rate inversely proportional to the input reactance X_AT of the line. The resistive part of the line impedance R_AT causes a reaction pressure which is also a component of p. ∑_AT and _AT are frequency dependent line characteristics. The front chamber pressure also causes a reaction force on the diaphragm equal to p times S_D. This is another component of F_D.

Now, assume that the inner vent 5 is no longer blocked. The pressure in the back chamber _B will drive a flow through the inner vent with a volume velocity U_p which is equal to the integral of the pressure p_B with respect to time divided by the acoustic mass of the vent M_Ap. The volume velocity components U_D and U_ now add to form the total flow into the front chamber U_p which, in turn, drives the system output U_Q.

In the arrangement of FIGS. 3 and 4, the analysis of the system is similar, except that the line impedance is simplifed because the short tube presents a lumped parameter element. In this case, the output flow U_Q is equal to the front chamber pressure p_F integrated with respect to time and divided by the acoustic mass of the outlet vent M^ . The opposing pressure component of _F results from the flow losses in the outlet R_j^p.

Analysis of the signal flow graphs yields the approriate design equations which allow the correct driver and enclosure parameters to specify for a desired system.

FIGS. 5 to 8 show views of a practical loudspeaker system using the present invention which has particular application in active noise control systems. In this apparatus an additional component, a flow tube 7 for the noisy flow (such as the exhaust of an engine) , has been added. Also, a drain tube 8 has been added between the front and back chamber so that water or other liquids trapped in the back chamber may escape. If the loudspeaker were used in an active noise cancellation system on a vehicle and if the vehicle were driven through deep water, the muffler could be flooded. The drain tube would allow the trapped water to flow out of the back chamber. The drain tube must be sized so that it acts as an acoustic mass rather than an acoustic leak between the chambers. Its mass must either be considered when adjusting the enclosure tuning or be trivial compared to the inner vent 5 so that the effect of the drain may be ignored.

FIG. 9 shows an apparatus using the present invention which also has particular application for active noise control. In this instance, the short tube 6 is formed by the area between the heat shield plate 7 and the connection to the noisy duct 8. A long, narrow tube 9 allows outsider air to enter the enclosure. This tube, like the drain tube discussed above, should be sized so that it has no adverse effect on the system acoustic performance. It may enter the enclosure through either the front or back chamber. Air is forced through the system because of the venturi-like detail 10 in the noisy duct. The flow through the duct over the "venturi" cause a low pressure region which "draws" the outside air. This air may be useful for cooling or removal of corrosive gases. The analysis and derivation of the analog circuit of the Vented Box Bandpass Loudspeaker is as follows: The symbol used in Figures 10 and 11 and in the calculations are: in Figures 10 and 11 and in the calculations are:

LIST OF SYMBOLS

^CAB Acoustic compliance of Rear Box

^CAP Acoustic compliance of Front Box C_AS Acoustic compliance of Driver (Loudspeaker ^VAS^=P _O ^C2C S^

^MAP Acoustic mass of Front Port

^MA_S Acoustic mass of Driver

^M _AB Acoustic mass of Internal Port

R_AS Acoustic Resistance of Driver R_E Electrical Resistance of Driver voice coil

S_D Driver Diaphragm, M²

V_B Volume of Rear Closed Box (M³) (V_B=P₀C²C_AB)

V_p Volume of Front Box (M³) (V_p=P₀C²C_Ap)

V_d Peak displacement volume driver diaphragm (S_DX_M) P_Q Mas densily of air (7.18 kg/m³)

C Speed of sound in air (345 m/sec)

X-_j. Peak linear displacement of driver diaphragm

S_p Area of the front port

S_B Area of the internal port B Magnetic flux density in driver airgap

1 lenght of voice coil in the airgap of driver

U_Q Volume velocity at the front port

^UAB ^v°l^ume velocity at the internal port

U_F Volume velocity inside the front box (Up=U_s+U_AB) U_β Volume velocity inside the rear box (U_B=-U_F) -

U_s Volume velocity generated at the source

P_ Pressure generator (equivalent)

E„ Input voltage to the loudspeaker

Speaker Parameters η f_s (T_s= —~ —) Free Air Resonance frequency

Q_ES Electro-Magnetic Q at f_,

Q_ms Mechanical Q at f_s

^Qts ^{Total Q at f}s| ^Qe ^Qms

^Qes^+Qms Vas Volume of air having sam acoustic compliance as driver suspension Vd Peak displacement volume of diaphragm (=S_DX_M) S_D Effective diaphragm area Xm Peak linear displacement of diaphragm

Referring now to Fig. 10, there is shown the general form of the Vented Box Bandpass Loudspeaker (VBBP) configuration.

Fig. 11 shows the simplified acoustical analagous circuit of the Vented Box Bandpass Loudspeaker (VBBP) configuration. The terms R_Q and P are determined by the following formulal:

R₀ = ^(BL) P_g = V^BL) pS_D RS_D

In the following circuit analysis, assumptions are made that there is a lossless enclosure (internal box resistance =

O and leakage resistance =α) and that the voice coil inductance is small (Lp«0) The circuit analysis is as follows:

1 1 -_ . U_τ

(1) P_r (^Ro^+RA_S) ^+SMAS⁺- ^Us⁺- __ +sM ^LA_Pτ-U^uo sC ^•AS^J sC AS

^UF^"Uo

(2) = ^ΞM_Ap-U₀ sC_Ap

(4) From Equation (2 ) U_F= (l+S²M_ApC_Ap) U₀ U_S+U_AB=U_F (5) U_S=U_F-U_AB

1 ^1+sMAP^CAP ,

From equation (3 ) U ^JA_ΛTB₃.=^~- — I + ^τs^βM^jaA_ΛP| U ^υo ^sMAB ^{■ sC}AB ⁺- .( l+β (M_ApC_Ap+C_ABM_Ap) b₀ 5^ΛM A_AB_IΪC'-AB

Substitute Equations (4) and (6) into the Equation (1)

⁺ S²(^MAB^CAB ^{+ M}AP^CAP ^{+ M}AP^CAB> ^{+ 1}

(1 + ^S'^MAP^CAP>

(8) P_g(S³C_ASM_ABC_AB)={S⁶M_ASC_ASM_ApC_ApM_ABC_AB+

^S4MAS^CAS^MAB^CAB ⁺ ^S4MAS^CAS^MAP^CAP^+S4MAS^CAS^MAP^CAB

+S M_ASC_AS+S C_ASR_AtM_ApC_ApM_ABC_AB+S C_AgR_ATM_ABC_AB

^+S ^S^T^AP^^{"1"8 C}AS^RAT^MAP^CAB^+SCAS^RAT

+ S⁴M_ApC_ApM_ABC_AB+S²

+l₊S²M_ABC_AB+S C_ApM_ABC_ABM_Ap+S⁴C_ASM_ApM_ABC_AB}U₀

(9 ) Pg (S³C_ASM_ABC_AB) ={S⁶M_ASC_ASM_ApC_ApM_ABC_AB-.

^s5cAS^RAT^CAP^MAB^CAB^{+ S} <^MAS^CAS^MAB^CAB^+MAS^CAS^MAP^CAP⁺

^MAS^CAS^CAB^+MAP^CAP^MAB^CAB^+MAB^CAB^MAP^CAP

^+M _AR ^C _AR ^M _AP ^C _A._.)+S^JC_ARR_AT(M_ApC_Ap+M_ApC_AB+ ^MAB^CAB>^+s2 <^MAS^CAS^+2MAB^CAB^+MAP^CAP^+MAP^CAB)

^+SCAS^RAT⁺¹>^Uo

ASSUMPTIONS

(10) M_ASC_AS

(11) M_ABC_AB

^MAP^CAP

(13) M_ApC_AS

(14) M_ApC_AB

P_C^< p₀c

(15) R_AT=R₀+R_AS = +

^Ws^VAS°__S ^WS^VAS^QMS ^CAS s°-ts

(16) C_{AS AT}

(17) P (S) = sM_ApU₀

SYSTEM TRANSFER FUNCTION

^sMAP^ϋO ^Pθ(^s)

(18) G(s) = = P_g P_g(s) bs⁴

(19) G(s) =

S^b + a₅S + a₄S⁴ + a-S + a₂S^;,:: + a^¹ + a_Q when, τ_B ²τ_p ² i τ_B ²τ_p ²τ_s 1 _s

(20) a₅= x ^W _S°-ts T_s ²T_B ²T_p ² _s ² _B ²T_p2Q_ts T_sQ_ts Q_ts (21) a, = (τ_s ²τ_B ²+τ_s ²τ ²+T_.²T. 6 i om __ m ^_ι_rπ ώm _ s -pB ^¹p ¹B ⁺¹B ¹ps

^■B

^<23) "² τ_B <^TS^22TB^2+T _P ² ₊ ^T-_B ⁾-

=(W_B ²W ²+2 _s ²W ²+W_s ² _B ²+

(25) a_Q = , = W_{S B} ²W m <__rn ώrn __. ^

₍₂₆₎ b= ^MARCASMABCAB s^2τB² ₌ ^Tps² ₌ ^Ws^2wp²

W W T^T^ w_p£

Claims

CLAIMS 1. Apparatus for achieving improved performance of a transmission line loudspeaker producing a front and back wave comprising one or more electromagnetic driver means mounted in an enclosure means having front and back chamber means connected by a conduit means so that the back wave of the loudspeaker is constructively summed with the front wave in the front chamber means, and a transmission line means attached to the front chamber means comprising the only sound outlet of the system.

2. Apparatus as claimed in claim 1, in which the transmission line means is a short tube means in the enclosure configured to produce a desired band-pass frequency response characteristic.

3. Apparatus as claimed in claim 2, in which a separate pipe means passes through the loudspeaker enclosure means and exits through the short take means for the purpose of noise cancellation.

4. Apparatus as claimed in claim 1, in which a drain means is added between the from and back chamber means to allow trapped liquids to drain from the back chamber.

5. Apparatus as claimed in claim 2, in which a drain tube means is added between the front and back chamber means to allow trapped liquids to drain from the back chamber. 6. Apparatus as claimed in claim 3, in which a drain hole is provided between the front and back chamber means to allow trapped liquids to drain from the back chamber.

7. Apparatus as claimed in claim 2, including a noisy duct means to which the short tube means is connected for the purpose of noise control.

8. Apparatus as claimed in claim 7, in which the short tube means is formed by the area between a plate means in the front chamber means and the connection to the noisy duct or pipe.

9. Apparatus as claimed in claim 7, in which a ventilation tube means is connected between either the front or the back chamber means and the outside air and a venturi-like structure means is in the noisy duct means in order to create a low pressure region to draw outside air through the system.

10. Apparatus as claimed in claim 1 in which said conduit means comprises a hole connecting said front and back chambers.

11. Apparatus for achieving improved performance of a transmission line loudspeaker means producing a front and back wave comprising a driver means, said driver means mounted in an enclosure means, said enclosure means having front and back chamber means connected by a conduit means so that the back wave of the loudspeaker means is constructively summed with the means attached to the from chamber means comprising the sole sound outlet of the system.

12. Apparatus as claimed in claim 11 in which the transmission line means is a short tube means in the enclosure configured to produce a desired band-pass frequency response characteristic.

13. Apparatus as claimed in claim 2, in which a separate pipe means passes through the loudspeaker enclosure means and exits through the short tube means for the purpose of noise cancellation.

14. Apparatus as claimed in claim 1, in which a drain means is added between the front and back chamber means to allow trapped liquids to drain from the back chamber.

15. Apparatus as claimed in claim 2, in which a drain tube means is added between the from and back chamber means to allow trapped liquids to drain from the back chamber.

16. Apparatus as claimed in claim 3, in which a drain hole is provided between the front and back chamber means to allow trapped liquids to drain from the back chamber.

17. Apparatus as claimed in claim 2, including a noisy duct means to which the short tube means is connected for the purpose of noise control. 18. Apparatus as claimed in claim 7, in which the short tube means is formed by the area between a plate means in the front chamber means and the connection to the noisy duct or pipe.

19. Apparatus as claimed in claim 7, in which a ventilation tube means is connected between either the front or the back chamber means and the outside are and a venturi-like structure means is in the noisy duct means in order to create a low pressure region to draw outside air through the system.